Conventionally this type of positioner is designed so as to operate with a DC electric current signal between 4 and 20 mA (a control signal) sent through a pair of electric wires from a higher-level device. For example, if a current of 4 mA is sent from the higher-level system, the opening of the regulator valve is set to 0%, and if a current of 20 mA is sent, then the opening of the regulator valve is set to 100%.
Rapid responsiveness is important in industrial process control systems, such as oil or gas processing systems, or the like, wherein a number of such regulator valves are installed. That is, it is important for there not to be hidden leaks or uncontrolled discharges of toxic or flammable chemical substances, gases, or the like.
Because of this, it is desirable for a regulator valve to have a function to shut down a valve at the time of an emergency, and for this purpose, positioners for controlling the degrees of opening regulator valves are designed to have such emergency shutdown functions.
In recent years intelligent positioners, wherein the positioners are equipped with microprocessors, have become popular. Such intelligent positioners, in addition to software for controlling the degree of opening, also have software for an emergency shutdown function, enabling full closure of the valve when there is an emergency.
For example, the value of a control signal that is sent from a higher-level device is monitored through a processing operation of a microprocessor following the software for controlling the degree of opening, and if it is confirmed that the value of that control signal has fallen below a prescribed setting value, then the valve is forcibly closed completely.
However, it is not particularly desirable the production of this function at the time of an emergency shutdown to be through such software. This is because validation of the reliability of digital circuitry, and the like, associated with the microprocessor for executing the software, and of the software itself, is costly and time-consuming. Moreover, because the processing operations following the software are complex to begin with, the risk increases with the inability to tolerate the failure rate of the emergency shutdown achieved through such a processing operation.
Given this, in, for example, Japanese Examined Patent Application Publication 2007-512622 (“JP '622”), an example is presented wherein the positioner itself is provided with a circuit for the emergency shutdown, rather than relying on software. In the electropneumatic controller (positioner) set forth in JP '622, a shutdown unit is provided as a circuit for an emergency shutdown between the electropneumatic converter and a control unit that is structured from a microprocessor or a digital circuit.
FIG. 5 illustrates a positioner provided with a shutdown unit between the control unit and the electropneumatic converter. In this figure, 100 is a higher-level device, 200 is the positioner, and 300 is a regulator valve. The positioner 200 is provided with a control unit 1, an electropneumatic converter 2, a pilot relay 3, and an opening sensor 4, where a shutdown unit 5 is provided between the control unit 1 and the electropneumatic converter 2.
In this positioner 200, when a control signal (a DC electric current signal) of between 4 and 20 mA is sent from the higher-level device 100, the control unit 1 inputs an valve opening signal that is fed back through an opening sensor 4 from the regulator valve 300, and calculates a deviation between the value indicated by the control signal (a setting opening θ sp) and the value indicated by the valve opening signal (the actual opening θ pv), to continuously vary the value in accordance with this deviation, to thereby output an adjusted electric current signal Io as an adjusted electric current. A control transistor Q1 is used in adjusting the electric current in the control unit 1.
The electropneumatic converter 2 inputs an electric current signal (the adjusted electric current) Io from the control unit 1 through the shutdown unit 5, to convert the inputted electric current signal (the adjusted electric current) Io into a pneumatic signal Pn. Note that in the electropneumatic converter 2, the electric current signal (the adjusted electric current) Io from the control unit 1 is supplied to a magnetic excitation coil, to adjust the distance of separation between a nozzle that releases the supply air and a flapper, to output, as the pneumatic signal Pn, the nozzle back pressure obtained thereby. The pilot relay 3 amplifies the pneumatic signal Pn from the electropneumatic converter 2, to supply it to the operating device 7 of the regulator valve 300 as the output pneumatic pressure Pout.
The shutdown unit 5 comprises a comparing circuit 51 and an electronic switch 52 through an electronic device such as an FET, where the electronic switch 52 is provided in the route for supplying the electric current signal (the adjusted electric current) Io to the electropneumatic converter 2 from the control unit 1. In the shutdown unit 5, the control signal from the higher-level device 100 to the control unit 1 is branched and inputted, where if the value of this control signal that is branched and inputted is less than a prescribed threshold value, then the electronic switch 52 is turned OFF, to shut down the supply of the electric current signal (the adjusted electric current) Io from the control unit 1 to the electropneumatic converter 2. Doing so reduces the output pneumatic pressure Pout from the positioner 200 to essentially near zero, causing the regulator valve 300 to be closed forcibly.
In the positioner, the adjusted electric current that is supplied to the electropneumatic converter is important because it controls the degree of opening of the regulator valve. There can be a risk that the plant could be shutdown if there was an unlikely interruption of the supply of the adjusted electric current to the electropneumatic device in a state wherein no shutdown should be performed. For example, if there were a broken line in the loop of the adjusted electric current that is supplied to the electropneumatic converter, then the supply would be stopped.
However, in the positioner 200 that is illustrated in FIG. 5, the electronic switch 52 is disposed in the loop for this adjusted electric current, to achieve the shutdown if the electronic switch 52 is turned OFF. In such a structure, there is the risk that the electronic switch 52 may become damaged due to a shock or impact from, for example, noise, due to a defect therein, or through becoming worn, so that the existence of this electronic switch 52 becomes a factor that may trigger an unnecessary shutdown notwithstanding the operating conditions being normal. Moreover, it is necessary to add this electronic switch 52 between the control unit 1 and the electropneumatic converter 2, which increases costs.
The present invention solves such problems, and the object thereof is to provide a positioner that can produce an emergency shutdown function without the provision of an electronic switch in the adjusted electric current loop and without relying on software.